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Part of Target3D's Virtual Production Explainer Series
Pixels or “picture elements” are tiny components that make up digital images. In traditional filmmaking they aren’t overly important, however, with the development of Virtual Production pixels become far more significant. They may be small, but they have the ability to make or break a shoot.
If you’ve ever tried to take a photo or video of a computer screen, you’re probably familiar with what’s known as “moire”. Moire is an unfortunate side effect that becomes apparent when capturing certain patterns with a camera. When grid lines, formed by the placement of pixels on an LED panel, are misaligned with the imaging sensor in a camera it causes a glitchy pattern that often obscures the content on display. Moire is something that most people have encountered, and it gives us a great entryway to the minutiae of filming with LED walls.
We’ve covered what Virtual Production is in our previous explainer, and explored how VP using LED walls and a camera allows filmmakers more freedom to create. However, this comes with caveats. Pre-production time is longer in VP shoots compared to traditional filmmaking, as it takes time and attention to get the image to appear at its best. This begs the question - how are VP filmmakers shooting directly at LED screens without breaking the illusion?
Tiny spaces between the pixels forming a line is enough to trigger the moire effect. Overcoming this is in part to do with what we call pixel pitch. Technically, this refers to the distance from the centre of one pixel to another. This translates to the distance that a camera can capture an LED wall whilst still retaining its fidelity or the illusion of a single, unbroken image rather than capturing a set of pixels.
Pixel pitch determines how close the performer and camera can be to the wall before focus falls onto the wall rather than the subject. Filmmakers need to consider the size of the space available, the number of actors in the volume, the movements actors will make and the camera angles necessary to capture the performance.
Therefore pixel pitch must be calculated to achieve the best picture. A higher pixel pitch means that there’s more distance between pixels and a lower resolution. As you would expect, a lower pixel pitch means a smaller distance between pixels and a higher resolution. Pixel pitch is abbreviated by millimetres, meaning that 1 mm pixel pitch is “P1”. It can go as high as P40, with this size used solely in outdoor settings.
A lower pixel pitch comes with an increased cost and the opposite is true for higher pixel pitch. This is why it’s vital to examine the space before choosing LED panels, as high pixel pitch panels will not allow for extreme close ups to the wall whilst maintaining the fidelity of the image. The average vision limit per 1 degree is 60 pixels.
Finer pixel pitches come with other caveats to consider as well as moire - scan mux ratio is an essential factor. This is how many pixels are driven by a single IC driver. An LED pixel is either on or off, the brightness is determined by how quickly it flashes on or off known as PWM (pulse width modulation). In order to improve performance at low brightness and greyscales the pixels must refresh faster. The optimum ratio is between 1:6 and 1:11, above this drastically increases the chances of artefacts. Sub 1.5mm pitches begin to increase ratios again due to a lack of physical space, cost and heat dispersion with emerging technologies such as flip chip micro LEDs looking to answer this.
Working around moire can be as simple as shooting with a shallower depth of field. This sets focus onto the subject softening the focus on the wall if shooting ICVFX. However, this is highly dependent on the creative vision of the filmmakers and may not be suitable in all cases, such as when shooting XR.
The type of LED used can also factor into how well VP works for a production. A standard 3-in-1 SMD pixel array gives more chance of moire artifacting, which can be counteracted using an anti-moire mask as well as colourshift. Conversely, the need for this is removed by using a mini LED pixel. This is because the 4-in-1 array is maskless, removing the mask grid and increasing the viewing angle. However as always, there is a price difference depending on what hardware is used.
High pixel pitch shouldn’t be a deterrent with VP as they have a key role for ICVFX to create dynamic light from ceiling and roaming totems. Pixel pitches from 3.9 and above have significantly higher brightness capabilities. Most LED ceilings are currently 5mm and offer a brightness of around 5000 nits, though this pitch is continually decreasing. It is essential that the demands of the volume are kept in mind. As long as filmmakers understand how to effectively use LED VP volumes, the possibilities are endless. If you’re interested in learning more about Virtual Production compared to traditional production, take a look at our previous article on what VP does for traditional filmmakers.
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